Condensate polishing is a specialized application for ion exchange resins, in which these resins are used to treat the water from which steam is generated to drive steam turbines. Commonly, the generating system is a closed cycle, in which the steam is condensed, polished by passing it through the ion exchange bed, and vaporized again to operate the turbine. Purity requirements for such waters are stringent, and levels above a few parts per billion (ppb=1.times.10.sup.-9 g/g) sodium ion, for example, are considered unacceptable. The condensate polishing bed of ion exchange resins is intended to maintain the undesirable impurities below levels such as these, especially in the event of a leak in the condenser cooling system, which might permit raw cooling water to enter the closed, high-purity, steam-generating system. Such leaks will challenge the system with ionic impurities at levels typically on the order of a few hundred ppb. In addition, the cation exchange resin of the condensate polisher must control the level of sodium introduced during regeneration of the ion exchange resins. During the separation that precedes regeneration of mixed-bed condensate polishers, a small amount of the cation exchange resin is unavoidably entrained with the anion exchange resin, and is exhausted with sodium during the anion exchange regeneration with sodium hydroxide. Some of this sodium will leak into the condensate, and must also be controlled to the requisite level.
The resin for these stringent requirements for water purity is to protect the system, especially the turbine, from corrosion. For this reason, also, ammonia is added to the steam generator feed water, to adjust the system to a slightly alkaline pH. The levels of ammonia used are quite low, typically from a few hundred to a few thousand ppb ammonium ion. The cation exchange resin in the ion exchange resin bed is thus presented with a difficult task: it must allow the few thousand ppb ammonia to remain in the condensate, but must remove any excess metal ions down to the required purity level.
Cation exchange resins which heretofore have typically been used in this application operate initially in the "hydrogen cycle". That is, upon regeneration, the cation exchange resin is in the hydrogen ion form, and upon exposure to ammonium ions in the condensate water, it will exchange the hydrogen ions for the ammonium ions, lowering the concentration of ammonium ions and lowering the pH of the condensate water. As ammonia continues to be added to the condensate, all of the hydrogen-form cation exchange resin will be converted to the ammonium form, and the ammonium ion begins to break through the bed at approximately the input concentration, signaling the beginning of the "ammonia cycle" operation.
Sodium ions are present in the condensate system; most often these are introduced during regeneration of the resin bed, when a small amount of the cation exchange resin is carried into the anion exchange resin regeneration vessel and exposed to the sodium hydroxide regenerant. Additional sodium may be present from water leaks or other sources. The cation exchange resins which have been used in the past for condensate polishing leak relatively little sodium during the hydrogen cycle, and little difficulty is met keeping the sodium concentration below the required limit. At the establishment of the ammonia cycle, however, the concentration of sodium ions in the condensate, i.e., the sodium leakage, is no longer controlled by the sodium-hydrogen equilibrium, which strongly favors adsorption of sodium. The controlling equilibrium becomes the sodium-ammonium equilibrium, which favors adsorption of ammonium ions about equally with sodium ions. The sodium ion leakage will continue to be low in absolute terms, but it will sharply increase over the leakage observed during the hydrogen cycle, and this increase is often enough to exceed the sodium ion leakage specification for the condensate system.